This invention relates generally to chlorine analysis, and more particularly to the amperometric measurement of free chlorine residual.
Chlorination is widely used to purify water supplies. In practice, chlorine is introduced at a selected point in the water supply system, and flow then takes place into a tank or through a region of flow which is sufficient for the chlorine to act effectively on the contaminants present in the water to produce a disinfecting action. In order to determine whether the amount of chlorine present is adequate to effect disinfection, measurements are made beyond the chlorine input point. The measurement output signal may also serve to regulate the feed of chlorine into the system to insure that the amount is adequate but not excessive.
The amount of chlorine added to the water is referred to as the "dosage," and is usually expressed as parts per million (ppm). The amount of chlorine used up or consumed by bacteria, algae, organic compounds and some inorganic substances, such as iron or manganese, is designated as the "demand." Since many of the reactions with chlorine are not instantaneous, but require time to reach completion, chlorine demand is time-dependent.
The amount of chlorine remaining in the water at the time of measurement is referred to as the "residual." Residual is therefore determined by the dosage subtracted from the demand. Inasmuch as chlorine demand is time-dependent, this dependency is likewise true of chlorine residual.
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acids is formed. Actually, the hydrochloric acid always completely dissociates into hydrogen and chloride ions, whereas the hypochlorous acid only partially dissociates into hydrogen and hypochlorite ions. In either the hypochlorous acid or hypochlorite ion form, chlorine is called "free chlorine residual." Free chlorine residual has a highly effective killing power toward bacteria.
Should the chlorinated water contain ammonia or certain amino (nitrogen-based) compounds, as is the case with sewage, then addition compounds, called chloramines, are formed. Chloramines occur almost instantaneously, and though several reactions are possible between hypochlorous acid and ammonia, chloramines collectively are referred to as "combined chlorine residual." This combined chlorine residual has a much lower bactericidal effect than free chlorine residual.
When sufficiently high chlorine dosages are applied to waters containing ammonia, different reactions will occur, resulting in the destruction of the ammonia and the formation of free chlorine residual. Thus, for water containing a known amount of ammonia, if one starts with a chlorine dosage which is low, chloramines will be formed resulting in a combined chlorine residual whose bactericidal effect is relatively weak. And as the dosage is raised, the amount of combined chlorine residual produced also increases, until a peak is reached when all of the free ammonia is used up in the formation of chloramine. And as the dosage is elevated beyond the level at which the combined chlorine residual peaks, destruction of the chloramines, which are unstable, takes place until a breakpoint is reached indicating that chloramine destruction is at its maximum. At breakpoint, the first appearance of free chlorine occurs.
A further increase in chlorine dosage beyond this breakpoint results in the formation of free chlorine residual. Thus by using a chlorine dosage sufficient to attain the breakpoint state, one is able to get rid of virtually all ammonia and a majority of chloramines. It is to be noted that complete destruction of chloramines seldom occurs at breakpoint, and some chloramines invariably persist in the presence of free chlorine.
Domestic waste water is typically high in ammonia, the ammonia resulting primarily from hydrolysis of urea. Normally, almost all of the nitrogen formed in solutions that enter a waste treatment plant are in the least oxidized, ammonia form. In conventional secondary waste treatment, a portion of the ammonia will be completely nitrified to nitrate, some will be only partially nitrified to nitrite and a portion will remain as ammonia.
Although some nitrogen is removed from domestic waste water by means of secondary treatment, the percentage of removal is usually only twenty to fifty percent. The remaining nitrogen is discharged from the treatment system into a receiving body of water with adverse ecological results, for nitrogen discharged as ammonia or nitrite exhibits an oxygen demand on the receiving body and creates a chlorine demand at downstream water-treatment plants. Moreover, the nitrate form contributes significantly to eutrophication or algae bloom in streams and lakes.
As pointed out previously, chlorination of sewage plant effluents in the presence of ammonia gives rise to chloramines, which is a far less effective disinfectant than free chlorine. Breakpoint chlorination has therefore been used to destroy ammonia, but it has not heretofore been possible to adequately control the technique involved.
The obvious way to govern the process is to use conventional feedback control means based on free chlorine residual, for the breakpoint at which ammonia destruction is optimized, is characterized by the first appearance of a free residual. But to utilize this approach, an analytical method must exist that is capable of accurately and continuously measuring free chlorine in the presence of substantial combined chlorine residual. Furthermore, it is of the utmost importance, should only combined chlorine residual be present, which is indicative that breakpoint has yet to be reached, that the continuous analyzer then indicate that essentially no free chlorine is present.
Existing amperometric cells used for the measurement of free chlorine residual are also responsive to combined chlorine residual (albeit less sensitive) and therefore produce a misleading reading of apparent free chlorine, thereby making effective breakpoint control difficult to accomplish.